A wireless transmission apparatus in a wireless communication system includes an amplifier that amplifies the power of a transmission signal. In the wireless transmission apparatus, the amplifier is generally operated in the vicinity of the saturation region of the amplifier to increase the power efficiency of the amplifier. However, if the amplifier is operated in the vicinity of the saturation region, nonlinear distortion increases. Hence, the wireless transmission apparatus includes a distortion compensation apparatus that compensates the nonlinear distortion to suppress the nonlinear distortion and reduce the adjacent channel leakage ratio (ACLR: Adjacent Channel Leakage Ratio).
One of distortion compensation methods used by the distortion compensation apparatus is a “predistortion (hereinafter may be referred to as “PD”) method.” A PD type distortion compensation apparatus previously multiplies a distortion compensation coefficient of the amplifier, which has an inverse characteristic of nonlinear distortion, by a transmission signal before being input into the amplifier. Accordingly, the linearity of the output of the amplifier is increased to suppress the distortion of the output of the amplifier. A signal after the distortion compensation coefficient was multiplied by the transmission signal may be referred to as the “predistortion signal (PD signal).” Hence, the PD signal is a previously distorted signal in accordance with the inverse characteristic of nonlinear distortion of the amplifier, before being input into the amplifier.
For example, one of known PD type distortion compensation apparatuses includes a table in which a plurality of distortion compensation coefficients is stored, and reads, from the table, a distortion compensation coefficient in accordance with the power of the transmission signal, in other words, the power input into the amplifier. The distortion compensation coefficients stored in the table are sequentially updated in such a manner as to minimize an error between the transmission signal and a signal output and fed back from the amplifier (hereinafter may be referred to as “feedback signal”).
Moreover, for example, one of known power amplification systems including the PD type distortion compensation apparatus limits a frequency band of the feedback signal upon updating of the distortion compensation coefficients. In the power amplification system, a passband width of the feedback signal is changed by a band-limiting filter to compensate predetermined lower-order distortion if the passband is a predetermined narrow width, and to compensate predetermined higher-order distortion if the passband is a predetermined wide width.
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2012-090158 and Japanese Laid-open Patent Publication No. 2012-060254.
FIG. 1 is a diagram provided for the description of the problem. If the transmission signal is a multi-carrier signal containing a plurality of signals at different frequencies from one another, when the amplifier is operated in the non-linear region for the multi-carrier signal, inter modulation distortion (Inter Modulation Distortion; hereinafter may be referred to as “IM”) may occur. For example, as illustrated in FIG. 1, if a multi-carrier signal containing a signal of a frequency f1 and a signal of a frequency f2 is amplified in the non-linear region, IM3, IM5, and IM7 being respectively third-, fifth-, and seventh-order IMs may occur. As in FIG. 1, these IMs occur at positions at both ends of the bottom of each of the frequencies f1 and f2, and at positions a fixed distance away from the frequencies f1 and f2, on the frequency axis. Here, the distortion compensation apparatus is assumed to be capable of compensating IMs up to IM7. In other words, a frequency band capable of distortion compensation (hereinafter may be referred to as the “distortion compensation band”) is assumed to be a band that covers up to IM7.
Here, the amplifier amplifies the transmission signal that has been converted from digital into analog by, for example, a DAC (Digital to Analog Converter; digital-to-analog converter) and then modulated by a quadrature modulator. Moreover, in this case, the feedback signal from the amplifier is demodulated by the quadrature demodulator, converted from analogue to digital by an ADC (Analog to Digital Converter; analog-to-digital converter), and input into the distortion compensation apparatus. The quadrature demodulator includes an analog filter to remove a folded component generated upon frequency conversion with the quadrature demodulator. Moreover, the DAC includes an analog filter to remove an image component generated by an interpolation process performed upon digital-to-analog conversion. The passbands of these analog filters are set according to the distortion compensation band. However, it is difficult to realize an analog filter as an ideal filter. Accordingly, frequency components at ends of the distortion compensation band are also cut off by these analog filters. For example, if the frequency response of the analog filters is one illustrated in FIG. 1, a part of IM7 lying in regions at both ends of the distortion compensation band is cut off. If the part of IM7 lying in the both end regions of the distortion compensation band is cut off, IM7 lying at the bottoms of the signals of the frequencies f1 and f2 is influenced so that the IM7 signal components are degraded. In this manner, if the feedback signal contains the degraded IM7 signal components, it is difficult to accurately perform distortion compensation on IM7. As a consequence, the accuracy of distortion compensation is reduced.
On the other hand, in the known power amplification system, consideration is not given to the problem in the reduction of the accuracy of distortion compensation, the problem having been caused by that a part of IM is cut off by the analog filter included in the quadrature demodulator or DAC.